The apparatus and process according to the invention relate to the cold forming of grooves on the revolving wall of a part by means of a pair of rotated knurling or milling wheels or rolls. These milling wheels have straight or inclined grooves on their peripheral wall, which are made to bear against the wall of the part, so that, by rolling, the wall undergoes a plastic deformation and in this way the corresponding grooves are obtained. The term rolling is generally understood to mean the application of a force to the rotating milling wheels, said wheels and their grooves sandwiching the revolving part on which forming is to take place, the part being rotated by friction with the milling wheels and their grooves. In general terms the grooves which it is proposed to produce thus make it possible to obtain shafts with parallel grooves or pinions having straight or inclined tooth systems, or threads for all types of applications.
Apparatuses or processes making it possible to obtain such grooves by rolling are already known. The GROB French patent 2 408 408 describes a process and an apparatus for forming by cold milling or knurling a tooth system on a cylindrical part rotating freely on a machine frame. This process relates to a machine of the type operating in remote manner between fixed axes, i.e. a machine whereof the axes of the shafts 8, 8' supporting the forming milling wheels 9, 9' are maintained at a constant equal distance from the axis of the revolving part to be formed throughout the forming operation. The two milling wheels 9, 9' have increasing radial tooth heights in the rotation direction so as to progressively penetrate the part 28. As has been explained in the above document, the teeth are produced in less than a revolution of the milling wheels, because the tooth profile of said wheels is evolutive. It is necessary to carry out a very precise angular keying or wedging of the milling wheels with respect to one another, so that the teeth of each wheel, during the rotation of the part, very accurately engage in the cavities formed by the teeth of the milling wheel located on the other side of the part.
This document describes means making it possible to obtain precision parts. It insists on the necessity of effectively keying the means supporting the bearings of the shaft, which drive the forming milling wheels 9, 9'. Such a keying or wedging by means of wedges or keys 15, 15' locked by screws 16, 16' bears on the frame of the machine 1. The synchronization of the shafts 8, 8' carrying the milling wheels is ensured by a pinion 21, which meshes on two other pinions 17, 17', each integral with one of the two shafts. A very precise angular keying of the milling wheels on the shafts has to be performed. The driving of the shafts of the milling wheels is brought about by two electric motors. A connection by articulated shafts 7, 7', provided with universal joints and sliding elements (not shown) makes it possible to adjust the distance between the milling wheel-holding shafts 8, 8', when this proves necessary.
Although this is not pointed out in the document, the remote forming apparatuses with a fixed centre-to-centre distance do not make it possible to produce precision parts, except when high quality evolutive profile milling wheels are used and which have a costly construction. The precise fitting of these milling wheels is relatively difficult and it is necessary to have a very considerable rigidity of the means supporting the shaft bearings, as well as the machine frame. Finally, these machines do not make it possible in general, to produce anything other than small parts. Thus, the necessity of having to produce the parts in less than one revolution of the milling wheel limits possibilities with regards to the production of larger parts.
Another process for the cold forming of grooves on the revolving wall of a part consists of using milling wheels having grooves or teeth with a constant profile instead of wheels having grooves or teeth with an evolutive profile. The penetration of the grooves or teeth of these milling wheels, with a constant profile and therefore a constant radial height, in the revolving wall of the part to be formed is brought about by the progressive reduction of the distance between the axes of a pair of milling wheels, rotating on shafts and between which is placed the revolving part to be formed.
The CHURCHILL German patent application 2025659 describes a device having two milling wheels 1, 2 mounted on shafts resting on fixed bearings at the front of two carriages 11, 12 able to slide on slides in order to very the distance between the axes of the milling wheel-holding shafts 1, 2 with a view to bringing the forming profiles thereof into contact with the revolving wall of the part 3 to be formed.
The moving together of the two carriages 11, 12, along an axis perpendicular to that of the part 3 to be formed, is brought about by two parallel jacks, whose cylinders 4, 5 are respectively located in the carriages 11, 12, whilst the heads of the pistons 6, 7 are fixed at the front of the opposite carriages 12, 11. Slides 8, 9 enable the two carriages to slide parallel to one another, so that the constant profile milling wheels 1, 2 come into contact with the part 3 to be formed. The penetration depth is controlled by means of abutments 26, 27 and their degree of screwing on the piston 6, 7 is adjusted. Thus, there is a limitation to the advance of the two carriages towards one another and therefore of the penetration depth of the profiles of the milling wheels in the wall of the part to be formed. It is also stated that it is possible to facilitate the engagement of the teeth of the milling wheels 1, 2 in the wall of the part 3 to be formed, by rotating said part 3 prior to the engagement of the milling wheels. It is also possible on the entire circumference of the part to form incipient grooves, e.g. by milling, so as to in some way guide the penetration of the teeth of the milling wheels 1, 2 in the wall of the part 3.
The SOUTHWESTERN INDUSTRIES INC French patent 2242174 also describes a method for forming grooves on revolving parts, by means of at least one milling wheel having a constant profile tooth system. The milling wheel is made to roll on the wall of the part, with a view to reproducing thereon the milling wheel tooth profile. In this method, at the time of contact between the toothed forming edge of the milling wheel and the wall of the part, the pressure exerted by the milling wheel must just be sufficient to rotate the part, whilst only marking thereon a line of the contour of said forming milling wheel. According to this patent, there is no need to simultaneously rotate the forming milling wheel or wheels on the one hand and the part to be formed on the other. It is merely necessary to rotate either the part to be formed, or the forming milling wheels. The initial pressure exerted by the wheel or wheels on the part during the rolling operation must be sufficient to rotate the part to be formed or the wheels if one or other thereof is free. This initial pressure must consequently be sufficient to rotate the part to be formed or the forming milling wheels without any relative sliding, for several wheel rotations, the final fixing only being subsequently obtained.
Although the aforementioned documents do not provide numerical examples, it is known that the cold forming of teeth or grooves on the wall of revolving parts by rolling by means of evolutive profile milling wheels gives better results with respect to the accuracy of the products obtained. This is explained by the use of a fixed centre-to-centre distance and an effective keying of the structure supporting the bearings of the milling wheels against a very rigid frame. The machines of this type are expensive in view of their very complicated, large structures and they are suitable for the production of large numbers of small parts. The considerable ratio which must exist between the diameter of evolutive profile milling wheels and that of the parts is also a factor opposing the construction of large parts by means of such apparatuses.
Apparatuses making it possible to cold form grooves or teeth by means of constant profile milling wheels have a much greater flexibility of use. Constant profile wheels are much less expensive to manufacture than evolutive profile wheels. The possibility of making their teeth or grooves penetrate the revolving wall of the part to be formed, whilst progressively decreasing the distance between the axes, makes it possible to very considerably reduce the pressure exerted. The same deformation work is carried out in a longer time during several revolutions of the milling wheels instead of less than a complete revolution. However, the displacement perpendicular to their axis of the milling wheel-holding shafts in order to move the wheels towards the part to be formed is an imprecision factor, which is added to the others and contributes to the obtaining of less accurate results. In the same way, the conditions under which the milling wheels come into contact with the part to be formed are less well defined.